1. Field of the Invention
The present invention relates to an electric motor vehicle having an electric motor mounted thereon as a driving power source, and particularly to an electric motor vehicle having a plurality of electric motors mounted thereon.
2. Description of the Related Art
Conventionally, there are known electric motor vehicles with one electric motor mounted thereon as a driving power source. With such electric motor vehicles, drawbacks are experienced in that there are limitations to the driving force, the efficiency is poor, and it is impossible to effect fine control according to a driver's requirements as in the case of four-wheel drive.
On the other hand, electric motor vehicles with a plurality of motors mounted thereon as a driving power source have been proposed, as shown in FIG. 9(a), (b), and (c).
FIG. 9(a) shows an example in which two motors are disposed in series to drive front wheels or rear wheels. FIG. 9(b) shows an example in which the front wheels and rear wheels are respectively driven by two separately provided motors. FIG. 9(c) shows an example in which the four wheels are respectively driven by four individual motors. In the drawings, reference numerals 51-62 denote wheels; 63-70, motors, and 71-73, differential gears. With such conventional arrangements, however, since control is effected by balancing the plurality of motors in such a manner that all of their outputs become identical, there has been a drawback in that the overall efficiency of the electric motor vehicle is poor.
To give a detailed description in this regard, first, the efficiency of a motor in general varies according to the rotational speed and output torque, as shown in FIG. 7. For example, if consideration is given to the efficiency when the vehicle is driven by two motors, as in the case of FIGS. 9(a) and 9(b), when the vehicle is running with each motor rotating at the rotational speed "A" in FIG. 7 and if the driver operates an accelerator pedal and requires torque "B" with respect to the vehicle, in a conventional vehicle, the two motors are controlled such that they output "B/2" of torque, respectively, so that the efficiency is only "X". However, if the torque "B" is within a range which can be sufficiently handled by one motor, if control is effected such that the output of one motor becomes "B" and the output of the other motor becomes zero, the efficiency becomes "Y(&gt;X)", with the result that the efficiency can be increased.
Particularly in the case of an electric motor vehicle which runs with one or more batteries mounted thereon, since the weight of the batteries is generally large, there is a drawback in that the traveling distance is short as compared with a gasoline engine vehicle. Accordingly, an increase in the efficiency of the vehicle has been a very important objective in the technical field of electric motor vehicles in the light of the reduction of the weight of batteries, an increase in the traveling distance, and so on.